Engineers faced the daunting task of slowing the Mars Exploration Rover spacecraft from about
12,000 miles per hour when they entered the atmosphere to about 12 miles per hour when they
hit the surface of Mars.

Better Parachutes and Airbags

A test of the Mars Exploration Rover parachute.

For entry, descent, and landing, the mission inherited much from its martian predecessors,
Viking and Mars Pathfinder. In order to slow the spacecraft down, the mission uses inherited
parachute technology from the Viking missions of the late 1970s, which translated well for 1997's
Mars Pathfinder mission. For the much heavier Mars Exploration Rover mission, the parachute's
basic design remained the same but is 40% larger in area than Pathfinder's parachute.

The same cluster of airbags that softened Pathfinder's landing also cushioned the Mars
Exploration Rovers. Twenty-four lobes that, when fully inflated, look like a massive bunch
of white grapes, surrounded the lander, with the rover cradled inside. The airbags are made
from a very durable, synthetic material called Vectran. On the human spaceflight side of NASA,
this same material is used to make spacesuits. Again, the weight increase from 1997's mission
called for a re-design of the airbags. Multiple drop tests proved that the additional mass
was causing serious abrasions and tears. Engineers designed a double-bladder airbag that
resisted these mission hazards, caused by the high speeds at which the rover package
lands and rocks and other geological features it may encounter.

The Opportunity rover undergoing final preparations before launch. The durable airbags are packed tightly against the tetrahedral lander.

Rocket Firings to Slow the Rover Down During Landing

This image, taken by the descent image motion estimation system (DIMES) camera located on the bottom of the Mars Exploration Rover Spirit's lander, shows a view of Gusev Crater as the lander descends to Mars.

To slow the rover package before it hits the surface, three rocket-assisted descent (RAD)
motors were designed for use if necessary. A radar system on the lander was able to detect
distance from the surface. When the craft were about 2.4 kilometers (or about 1.5 miles)
above the surface, the radar system alerted the Descent Image Motion Estimation Subsystem
(DIMES). This camera took three pictures of the ground about four seconds apart and automatically
analyzed them to estimate the spacecraft's horizontal speed. Moments later, a secondary
propulsion system that was new to the Mars Exploration Rover mission fired during Spirit's
descent. As predicted, Gusev Crater winds were fierce and moving Spirit from side to side,
impeding a safe landing. The Transverse Impulse Rocket System (TIRS) counteracts horizontal
movement, thereby making the craft more stable for landing. Windy conditions were not predicted
for Opportunity's Meridiani Planum landing site, so Opportunity did not fire its TIRS system.